1. Field of the Invention
The present invention relates generally to a system and method for transmitting control information in a mobile communication system, and in particular, to a system and method for transmitting uplink control information for performing fast cell switching in a communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
Mobile communication systems, originally developed to provide a voice service, are evolving into advanced systems capable of providing various multimedia services. The mobile communication systems are evolving into a 4th generation (4G) mobile communication system supporting high-speed multimedia services, following a 1st generation (1G) analog system, a 2nd generation (2G) digital system, and a 3rd generation (3G) IMT-2000 system that supports a high-speed multimedia service. In the 4G mobile communication system, a user can access a satellite network, a local area network (LAN), and an Internet network with one terminal, for example, one mobile station (MS). That is, the user can enjoy many kinds of services, such as voice, image, multimedia, Internet data, voice mail, and instant message services, with one mobile terminal.
The 4G mobile communication system aims at a data rate of 20 Mbps for a super high-speed multimedia service, and commonly uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
The OFDM scheme, a digital modulation scheme for multiplexing multiple orthogonal carrier signals, divides a single data stream into several low-speed streams and simultaneously transmits the low-speed streams using several subcarriers with a low data rate.
A multiple access scheme based on the OFDM scheme is known as an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. In the OFDMA scheme, subcarriers in one OFDM symbol are shared by a plurality of users, i.e. MSs. A communication system based on the OFDMA scheme (hereinafter referred to as an “OFDMA communication system”) has separate physical channels for transmitting uplink fast feedback information, which is a typical type of uplink control information.
The uplink fast feedback information includes full Signal-to-Noise Ratio (SNR) information, per-band differential SNR information, fast Multiple Input Multiple Output (MIMO) feedback information, and mode selection feedback information.
The uplink fast feedback information does not transmit large amounts of data as compared to the overall communication services. However, because the uplink fast feedback information is very important information for the communication system, highly reliable transmission should be guaranteed for the uplink fast feedback information. However, it is common that only a few of the frequency-time resources are allocated to physical channels, for example, fast feedback channels, used for transmitting the uplink fast feedback information, in order to reduce an overhead rate.
Generally, a combined method using a binary channel code and coherent modulation or differential modulation is used to transmit uplink control information. However, when the uplink control information is transmitted using less frequency-time resources then optimally required, an error rate increases, thereby decreasing operation stability of the communication system. While there are sufficient pilot tones for the transmission of downlink or uplink traffic, there are insufficient traffic tones for transmission of uplink control information. The lack of pilot tones deteriorates the channel estimation performance, thereby degrading the performance of a coherent modulation/demodulation scheme. If the number of pilot tones is increased taking into consideration only the channel estimation performance, the number of data tones becomes insufficient. In addition, separation of the binary channel code and the modulation causes some of the failures in performance. Further, if many of the frequency-time resources are used for transmitting the uplink control information, for example, uplink fast feedback information, in order to increase the stability, the overhead rate increases, which reduces throughput of the communication system.
The conventional method of transmitting uplink fast feedback information uses one uplink subchannel and transmits 4-bit information. However, the 4-bit information transmission cannot guarantee sufficient accuracy for the transmission of a full SNR and can transmit per-band differential SNRs only for 4 bands. In addition, the 4-bit information transmission lacks operation flexibility, so that is difficult to freely allocate codewords for the transmission of other information, as there are no more than 16 codewords.
In the existing cellular mobile communication system using a Code Division Multiple Access (CDMA) scheme, a fast cell switching (FCS) scheme is used to improve system performance. In the fast cell switching scheme, an MS manages several base stations (BSs) or several sectors in its active set, selects the best BS/sector having the best link performance from among the BSs or the sectors included in the active set, and informs the selected BS/sector of its best link performance through a Walsh code, in a process referred to as a data rate control (DRC) cover. The DRC cover refers to a Walsh code uniquely allocated to each BS in an Evolution Data Optimized (EV-DO) system. An MS sends a DRC cover request to a BS having a desired data rate, for example, the best DRC value and link performance, through a separate DRC channel. Thereafter, the MS receives downlink data from the BS/sector having the best link performance, thereby improving downlink performance. The active set is defined as a set of BSs or sectors that currently provide radio channels for the transmission/reception of the data to the MS. That is, an MS creates an active set with BSs, downlink performances of which are greater than or equal to a predetermined level, and BSs in the active set receive a variety of unique information on the MS. The foregoing scheme enables an MS located in a cell boundary to obtain selection diversity gain.
There is difficulty in directly applying the fast cell switching method to the OFDMA communication scheme. The CDMA scheme identifies many users, or MSs, using the user-specific long codes last multiplied in a digital transmitter. Therefore, every BS/sector can receive the signals transmitted by the MSs. Further, in the CDMA scheme, each BS/sector multiplies the received signals by the user-specific long code, and thereafter, despreads the multiplication result with Walsh codes corresponding to the MSs to determine if it is selected by the MSs as the best BS/sector.
However, in the OFDMA communication scheme, each BS/sector allocates frequency-time resources to a plurality of MSs, and the MSs must transmit signals only through the allocated frequency-time resources. If every BS/sector allocates independent frequency-time resources to all of the MSs employing the fast cell switching, this will serve as heavy overhead. If the information on the best BS/sector is transmitted not through a physical channel but through an upper layer message, the cell switching speed decreases, causing a decrease in selection diversity gain and scheduling performance.
As an existing scheme for solving the problems, there has been proposed a scheme for allocating a fast feedback channel for the transmission of channel quality information (CQI) and a fast feedback channel for the transmission of best BS/sector information, to an MS desiring to perform the fast cell switching. However, the existing scheme is inefficient because the MS requires two fast feedback channels in order to perform the fast cell switching.